1. Field of the Invention
The present invention pertains to an offshore tower structure, and to a method of installing that structure.
2. Discussion of the Prior Art
The development of offshore oil and gas fields has led to requirements that fixed drilling/production platforms be placed in deeper and deeper waters, thereby calling for taller, more costly support structures and the optimization of the useful life costs of those structures. In evaluating useful life costs, all phases in the life of a support structure need to be considered, including the site installation.
Conventional support structures have generally been fabricated as three-dimensional lattices composed of tubular steel members, otherwise known in the offshore industry as "jackets". Heretobefore, such jackets have been built to their full height in fabrication yards, either upright or, in the case of taller jackets, horizontally lying on one side face thereof. These tall jackets have been transported in one piece on barges, and then either launched or lifted into the water for upending and piling onto the seabed. Loads imposed by the launching and lifting operations, which constitute a transitory phase of installation, often need to be reacted by additional members designed for that sole purpose. Jackets which contain members designed solely for the transitory phase of installation continue to carry these additional members after installation and, consequently, they will be over-designed for the rest of their operational lives.
The recently developed "twin lift" installation technique for heavy structures has led to requirements for highly specialized, and consequently expensive, heavy lift crane vessels for lifting and upending jackets to be installed in deep water. Associated crane costs, and the costs of extra members required specifically for the installation phase, can be a significant part of the whole life cost of such a support structure.
Many known support structures have been designed to have sloping faces, so that the jacket tapers continuously from a large plan area at its base to a smaller plan area at its top. Tapering a jacket in this manner gives the structure a wide foundation to resist overturning moments, while its reduced section in the wave affected zone near the top of the structure attracts relatively low wave induced loads. Within the offshore industry, this concept of tapering is known as "batter". The requirement for batter all the way up a jacket has led to the design of some unwieldy structures which have had oversized members near their bases. To alleviate these structural inefficiencies, it has been proposed to build jackets with an enlarged base, a single tapered section, and a tower of uniform cross-section. One example of this type of jacket is shown in U.K. Patent Specification 2214548.
Conventional jackets and spread base jackets of the kind referred to above have been transported and installed in one piece. However, multi-part tower structures have also been proposed. One example is shown in U.S. Pat. No. 4,797,034 in which an unbattered upper section is mounted for limited compliant movement on top of a battered lower section which is fixed to the seabed. Another multi-part tower arrangement is illustrated in U.K. Patent Specification 1,491,684. A practical example of a multi-part tower is the HONDO structure installed off Southern California. This structure evinced complex joints between parts of the tower and substantial additional weight had to be designed into the structure to enable the connections to be made underwater. In this tower and the remaining prior art arrangements discussed above, the legs of the structures were aligned all the way up from seabed to above the sea surface.
Based on the above, it should be readily apparent that utilizing heavy lift vessels in installing offshore oil and gas towers significantly adds to the overall cast of the structures. Therefore, there exists a need in the art for a multi-part tower structure which can be installed without the need for expensive heavy lift vessels and which still realizes the advantages of conventional battered structures, particularly in deep water applications.